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Currently, direct imaging of\n                      <jats:sup>225<\/jats:sup>\n                      Ac\u2010emitted photons at low injected activity levels remains a technical challenge for image\u2010based\n                      <jats:sup>225<\/jats:sup>\n                      Ac dosimetry.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Purpose<\/jats:title>\n                    <jats:p>\n                      This study aims to assess the feasibility and accuracy of quantitative in vivo SPECT\/CT imaging for\n                      <jats:sup>225<\/jats:sup>\n                      Ac and its decay daughters at activity levels similar to those in small animal studies.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>\n                      A state\u2010of\u2010the\u2010art commercial preclinical SPECT\/CT scanner was used to image healthy nude mice in vivo and ex vivo after injection of unlabeled\n                      <jats:sup>225<\/jats:sup>\n                      Ac from the United States Department of Energy isotope program, at activities below 48.8\u00a0kBq. Each in\u2010vivo imaging was 1.5\u00a0h and ex\u2010vivo imaging was 12\u00a0h, at approximately 2\u00a0h after injection of\n                      <jats:sup>225<\/jats:sup>\n                      Ac, and all SPECT images were reconstructed at multi\u2010energy windows (82,\u00a0218,\u00a0and\u00a0440\u00a0keV). Mouse\u2010like phantom imaging for 1\u00a0h at 0.078 MBq and 12\u00a0h at 0.496 MBq were also performed to validate quantitative accuracy.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      In vivo SPECT\/CT images clearly showed abundant liver uptake, and significant kidney uptake was also observed in 440\u00a0keV images for\n                      <jats:sup>213<\/jats:sup>\n                      Bi, consistent with known renal toxicity from daughter redistribution. However, quantitative analysis of in vivo, ex vivo, and low\u2010activity phantom images revealed suboptimal quantitative accuracy, with high background recovery coefficients and variations in measured activity across the energy windows.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions<\/jats:title>\n                    <jats:p>\n                      We demonstrated the ability to image\n                      <jats:sup>225<\/jats:sup>\n                      Ac and its daughters in vivo at low activity levels. However, the quantitative results confirmed that, even with the state\u2010of\u2010the\u2010art preclinical SPECT\/CT system, current methods and reconstruction algorithms may not provide sufficient accuracy for accurate dosimetry at typical activity used in small animal studies.\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1002\/mp.70674","type":"journal-article","created":{"date-parts":[[2026,9,15]],"date-time":"2026-09-15T05:29:47Z","timestamp":1789450187000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Quantitative SPECT imaging of\n                    <sup>225<\/sup>\n                    Ac and its daughters in mice in vivo"],"prefix":"10.1002","volume":"53","author":[{"given":"Shixian","family":"Du","sequence":"first","affiliation":[{"name":"Department of Radiology and Biomedical Imaging University of California  San Francisco California USA"},{"name":"Department of Nuclear 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